One of the prior art electro-optical light modulating systems utilizes two identical electro-optic crystals positioned with the X-axis of one crystal and the Y-axis of the other crystal parallel and orthogonal to the light path, a modulating voltage being applied along the Z-axis of the two crystals to control the polarization of the transmitted light. Each of the crystals is a rectangular parallelopiped with square end faces.
In the prior art electro-optical modulator there is included two birefringent crystals having substantially the same angles to the optical axis which is determined by the coaxial alignment of one of each of the axes of the respective crystals. The modulation voltage is applied across a crystal with the electric vector parallel to the Z-axis of the crystal. The crystals are elongated and have a square cross section or a rectangular cross section.
It is noted that the usual crystal configuration is a rectangular parallelepiped. The optical wavelength, index of refraction, and crystal length determine the minimum crystal aperture which may be used without aperture effects. There exists an optimized beam periphery in a modulator configuration. This occurs only when the beam is confocally focused to a point between the crystals. The beam diameter, W, in this configuration is usually about one-third the crystal aperture size and the waist, W.sub.o, is .sqroot.2 smaller than W. No significant reduction of either the input or output crystal aperture is possible without impairing the modulator transmission. The maximum crystal capacitance which can be tolerated for adequate rise time sets the maximum useable crystal length. The special case of lithium tantalate being used for 0.53 mm operating at 1 Gbps limits the crystal length to about 10 mm. The minimum crystal aperture which can be used with two such crystals of this length is 0.22 mm resulting in a switching voltage of 22 volts for 150.degree. C. operation. Modulator driver capability for this application at the present time is not adequate for best performance at switching voltages above 20 volts.
Using rectangular parallelopiped crystals, there is no way of reducing the switching voltage for the above cases using two crystals without increasing the crystal capacitance for increasing the length and, hence, increasing the rise time too much for adequate 1 Gbps operation. The present invention by tapering the crystals provides a reduced switching voltage, or reduced capacitance, without reducing the aperture. The same optical safety factor is maintained in the entire crystal length for minimizing the drive voltage and power below that obtainable with straight cross section crystals.
The chief advantage of the tapered crystal modulator for this application is the capability of operating the modulator at reduced switching voltage and driver power. In addition, it is possible to taper the crystals for reduced crystal capacitance, thus decreasing the modulator rise time for improved response at a higher data rate.
It is emphasized that the present invention is an improvement in the usual crystal configuration for an electro-optical modulator for lasers wherein the configuration includes the aforementioned pair of crystals, each being a straight cross section crystal.
The chief advantage of the tapered crystal modulator for this application is the capability of operating the modulator at reduced switching voltage and driver power. In addition, it is possible to taper the crystals for reduced crystal capacitance, thus decreasing the modulator rise time for improved response at a higher data rate.
It is emphasized that the present invention is an improvement in the usual crystal configuration for an electro-optical modulator for lasers wherein the configuration includes the aforementioned pair of crystals, each being a straight cross section crystal.